It has been well documented that the progressive CD4+ T cell depletion and development of AIDS are closely associated to an HIV-1 coreceptor switch from CCR5 to CXCR4. The objective of this study is to investigate the basic mechanisms underlying CD4+ T cell depletion in AIDS using an SIV animal model system. We have constructed a cloned SIVmacEvT3 that uses CXCR4 as a coreceptor and rapidly induces CD4+ T cell depletion in rhesus macaques. In contrast, a cloned SIVmac239 that uses CCR5 does not induce CD4+ T cell depletion. The differences of coreceptor usage and pathogenic potential between EvT3 and 239 are determined by the gp120 sequences. In this study, we are investigating the basic mechanisms by which SIV CXCR4 usage contributes to CD4+ T cell depletion in vivo. In Year 1 of this grant, we established an RT-PCR method that quantitatively detects mRNA of CXCR4, CCR5 that function as coreceptors for EvT3 and 239, respectively. We found that CXCR4 was consistently expressed in rhesus CD4+ T cells at high levels in both resting and activated cells, whereas CCR5 expression was restricted to activated cells. This strongly suggests that the CXCR4 usage of EvT3 would facilitate infection of CD4+ T cells regardless of their activation status. We are currently investigating whether the SIV coreceptor usages coupled with the expression of coreceptors in CD4+ T cells determines the susceptibility of CD4+ T cells to SIV infection in both in vitro and in vivo systems. While EvT3 uses CXCR4 and induces CD4+ T cell depletion, it is not clear whether the CXCR4 usage is responsible for the CD4+ T cell depletion or other viral factor(s) in EvT3 gp120 contributes to the CD4+ T cell depletion. To identify genetic determinant(s) for CXCR4 use and pathogenic poten tial of EvT3, we constructed various recombinants and site-specific mutant viruses. Our data demonstrated that the EvT3 CXCR4 usage in vitro was primarily determined by the V3 sequences. However, preliminary data suggested that, in addition to the CXCR4 usage, an unidentified viral factor in EvT3 gp120 was required for the induction of CD4+ T cell depletion in vivo. Further studies will be warranted to understand the basic mechanisms underlying CD4+ T cell depletion and the role of viral coreceptor usage in the pathological event. The rhesus macaque model using the molecularly cloned SIV with a defined viral sequence, coreceptor usage and pathogenic potential will greatly contribute to better understanding of the significance of HIV-1 coreceptor usages for CD4+ T cell depletion in AIDS patients. This study will also potentially provide useful information for development of novel antiviral strategies that target to block the use of coreceptors by HIV-1. FUNDING NIH AI42508 PUBLICATIONS Martin K, Hagen S, Kodama T. Genetic determinants of SIV CXCR4 usage the role of V1 and V3 sequences. In 16th Annual Symposium on Nonhuman Primate Models for AIDS (held in Atlanta, GA, October 7-10, 1998) (abstract 3).